Cut the sweet potato into 1 cm dice and put into a bowl of water.,Drain.,Put the sweet potato in a pan with the soup cube and enough water to cover.,Simmer until there's almost no liquid left in the pan.,______,Just before all the liquid in the pan evaporates, add the komatsuna greens (still raw, not parboiled).,Simmer to evaporate the liquid.,Let it cool.,When it's cool, add the salt and pepper and mayonnaise and mix to finish.,You can mash the sweet potato until smooth if you like.,Delicious with some finely chopped crispy daikon radish leaves added.,This is really nutritious!